Conventional resistance temperature detectors (RTD), use a variety of materials to produce elements with high operating temperatures. These materials suffer from the detrimental effects of contamination, ionic migration, sublimation, oxidation and substantial decrease in mechanical strength with increased operating temperatures. Current temperature sensors are thus limited to an operating envelope of less than 650° C. (1200° F.) to ensure long term, stable output with minimum drift in resistance. Higher temperature devices can operate to temperatures up to 850° C. (1562° F.) but are either limited to specific environmental conditions (such as for instance: a vacuum environment, an inert gas environment, or a hydrogen atmosphere) and/or must be limited to short term operation to prevent premature failure. This temperature operating range has limited the application of these sensors in hostile, high temperature systems such as those commonly encountered in the aerospace, petroleum and glass industries.
Even though the temperature measurement conducted by employing a RTD is well known in the art, broad application of the RTD is still limited in high temperature hostile environments.
RTDs are useful temperature measuring devices which measure temperature by employing a variable resistant material at a point where the temperature is to be measured with lead ends connected to an instrument which measures the amount of varying voltage when power is supplied to the sensor. The resistant materials used for RTDs have been formed of various metals which provide a varying resistance upon exposure to varying heat.
Prior art temperature sensors have had the disadvantage of melting at fairly low temperature and have required insulation and various sheathing systems to protect the sensor during operation at prolonged elevated temperatures. However, this sometimes results in undesirable reactions between the metals in the temperature sensor and the materials used in the insulation and sheathing systems.
The problems of undesirable reactions in RTDs have been aggravated by the temperatures encountered in nuclear reactor systems, rocketry heat sensors, high-temperature and vacuum processing and other applications where temperature measurements at or above 1500° C. (2730° F.) are involved. RTDs have utilized sheathing and insulation in an effort to prevent the disintegration of the resistance material in such systems. The insulation and sheathing systems have the further disadvantage of resulting in time delays in obtaining temperature readings due to the insulation and mechanical packaging designed to prevent failure resulting from such problems as gas leakage at the RTD sheath seals, cracked sheaths and other mechanical limitations imposed by ceramic insulated metal sheathed sensors.
Platinum, being chemically stable and having high temperature dependency of electrical resistance, is employed as to a material for temperature sensors, and specifically, for RTDs. In a conventional platinum temperature sensor, a platinum wire is spirally wound on an insulator, or a platinum resistance pattern is formed as a thick or thin film on a substrate.
Other high melting, noble metals such as rhodium (Rh), palladium (Pd), iridium (Ir) as well as precious metals such as gold (Au) and silver (Ag), and alloys thereof are known in the art. Such metals, however, are not widely used because they are more susceptible to oxidation than platinum, and degrade by drift caused by selective oxidation.
Some of the characteristics of platinum can be improved by the usual alloy hardening method of adding a metal to the platinum base, followed by heat treatment. However, problems can occur after alloying. For example, when a high concentration of any alloying element is added to the platinum base, the electrical properties of the resulting platinum limb become inferior; at the same time the hardening phase will partially or totally dissolve into the base at high temperatures, thus the effects of the hardening action are reduced.
The prior art attempts to extend the operation range of variable-resistance temperature device have been limited to extending the range of known resistant materials through the use of insulation techniques or increasing the high temperature properties of known materials through alloying processes or coatings. The disadvantages of these techniques, including not reaching a high enough operating temperature, are discussed above. A significant benefit, however, is that the conversion of the output signal generated by the known resistant material is readily available through National Institute of Standards and Technology (N.I.S.T.) or International Electrotechnical Commission (I.E.C.) standard tables.
Conversely, if a resistant material was chosen based on its desired high temperature operating properties, and not based on providing a known resistance output, then higher operating range variable-resistance temperature device could be made, provided that the output signal of the resistant material is repeatable and convertible.
Dispersing oxides of transition metals or rare earth metals within noble or precious metals is an example of a method of creating variable resistant material with the desired extended temperature properties. For instance, dispersion hardened platinum materials (Pt DPH, Pt-10% Rh DPH, Pt-5% Au DPH) are useful materials because they achieve very high stress rupture strengths and thus permit greatly increased application temperatures than the comparable conventional alloys and are rugged.
Dispersion hardening (DPH) creates a new class of metal materials having resistance to thermal stress and corrosion resistance that is even greater than that of pure platinum and the solid solution hardened platinum alloys. When operational life, high temperature resistance, corrosion resistance and form stability are important, a sensor can be manufactured of DPH platinum and can be used at temperatures close to the melting point of platinum.
Dispersion hardened materials contain finely distributed transition element oxide particles which suppress grain growth and recrystallization even at the highest temperatures and also hinder both the movement of dislocations and sliding at the grain boundaries. The improved high temperature strength and the associated fine grain stability offer considerable advantages.
Platinum: Platinum-Rhodium Thermocouple Wire: Improved Thermal Stability on Yttrium Addition Platinum, By Baoyuan Wu and Ge Liu, Platinum Metals Rev., 1997, 41, (2), 81–85 is incorporated by reference. The Wu article discloses a process of dispersion hardening platinum for a platinum; platinum-rhodium thermocouple wire which incorporates traces of yttrium in the platinum limb.
As described in the Wu article, the addition of traces of yttrium to platinum as a dispersion phase markedly increases the tensile strength of the platinum at high temperature, prolongs the services life and improves the thermal stability. Yttrium addition prevents the growth in the grain size and helps retain the stable fine grain structure, as the dispersed particles of high melting point resist movements of dislocations and make the materials harder. The strength of a material is related to the movement and number of the dislocations.
In order to harden metals, the movement of the dislocations needs to be restricted either by the production of internal stress or by putting particles in the path of the dislocation. After the melting and annealing process, the majority of the trace yttrium (in the dispersion phase of the platinum) becomes yttrium oxide, which has a much higher melting point than platinum. When the temperature is near the melting point, dispersion hardened particles fix the dislocation, thus hardening the platinum and increasing its strength.
At the same time the grain structure becomes stable after dispersion hardening and there is also microstructural hardening. The dispersed particles affect the recrystallization dynamics, inhibit rearrangement of the dislocations on the grain boundaries and prevent the movement of the grain boundaries. Therefore, this dispersion hardened platinum possesses a stable fine grain structure at high temperature.
This patent outlines a variable-resistance temperature sensor with a protective sheath capable of protecting and extending the operating range of this class of sensor up to 1700° C. (3092° F.).